Nitrogen fixation is a process by which nitrogen gas is converted to heteroatomic molecules such as ammonia. Nitrogen fixation is carried out biologically by nature using enzymes, particularly nitrogenase containing Mo—Fe, V—Fe and Fe metals in the enzymatic catalyst or cofactors, although the detailed chemical mechanism remains unknown. Natural fixation is estimated to result in about 2×1011 kg of ammonia every year at ambient temperatures and pressures. Industrially, the chemical synthesis of about 8×1010 kg per year is produced by the Haber-Bosch process, which, although catalyzed by either Fe or Ru catalyst, requires very high temperatures and pressures. Hence, efforts have been made to identify catalysts that can mimic nitrogenase enzymes and fix nitrogen at near ambient conditions.
Although a few bimetallic catalysts have demonstrated activation of nitrogen, no catalytic bimetallic system has been identified. A monometallic catalyst has been identified, Yandulov et al. Science, 2003, 76, 301, that uses a tetradentate ligand with the early transition metal Mo where a turnover number of four has been observed. This catalyst is perceived as being active due to the ability of the ligand to inhibit formation of bimetallic complexes, to sterically protect the coordination site as a monometallic species, and increase the solubility of intermediates in nonpolar solvents. Hence, design of ligands and there use in organometallic compounds that promote stable nitrides of Mo or other similar transition metals may result in compounds for nitrogen fixation.
Pincer ligands are chelating agent that binds metals tightly to three adjacent coplanar sites. The pincer-metal interaction is very rigid and typically confers a high thermal stability to the ligand metal complexes. Organic portions and substituents define a hydrophobic pocket around the coordination site. These ligands traditionally share the common feature of an aromatic central aromatic unit. To this central unit are attached, in the ortho positions, two arms whose electronic and steric properties can be varied in many different ways. The ability to vary the properties of pincer ligands has been exploited for numerous complexes to be used as catalysis. Early work mainly focused on ligands where the central binding site is carbon and the peripheral binding sites are phosphorous, generally referred to by the atomic symbols of the donor atoms at the binding sites as the PCP systems. More recently CCC, CNC, CNS, NNN, NCN, PNP, OCO, SCS, SNS have been reported. Most frequently the pincer ligand transition metal complexes have been those of group VII-X metals where low coordinate and low oxidation state prevail and the metals are tolerant of a wide variety of substituents.
Early transition metal (group III-VI) pincer complexes are significantly less common and typically display high oxidation states and high coordination numbers, are typically electrophilic, and are intolerant of many functional groups. As most presently known pincer ligands have multiple soft donor atoms for metal binding, the ligands are not well suited to forming complexes with the early transition metals. Those that have been prepared include: pincer dicarbene complexes of CNC ligands with V, Ti, Cr, Mn, and Nb; non traditional NNN ligands with Zr; NCN ligands with W, Mo, Ti, La, Ta and Mn; and OCO ligands with Ti and Ta. The early transition metal form complexes with Pincer type ligands where the donors are all considered hard donors. As early transition metals, such as Mo, have been identified as promising for nitrogen fixation catalysts when sufficiently stable and bulky polydentate ligands are present, pincer complexes that can undergo nitrogen atom transfer from a complex are desired.